The invention relates generally to measuring the quality of an optical signal transmitted in an optical network. More specifically, the invention relates to systems and methods that measure an optical signal channel's signal-to-noise ratio using an interferometer in conjunction with a derived calibration factor ζ.
In high-speed optical networks based on Reconfigurable Optical Add-Drop Multiplexers (ROADMs) which are used to build dynamically reconfigurable Wavelength Division Multiplexing (WDM) networks, each signal channel may traverse through different routes, optical amplifiers, and add-drop filters. As a result of these different paths, noise levels of adjacent signal channels may vary. Inline optical filters used for demultiplexing the signal channels inside the ROADMs also suppress noise in-between the channels.
While ROADM-based networks are more economical, they present new measurement challenges for optical monitoring. An optical signal-to-noise ratio (OSNR) is the key performance parameter in optical networks for predicting the Bit Error Rate (BER) of a system. Typically, an Optical Spectrum Analyzer (OSA) is used to measure OSNR.
The International Electrotechnical Commission (IEC) defines the standard for measuring OSNR using a linear interpolation method. This method is based on the averaging of the noise levels to the left and right of a channel bandwidth peak leading to the out-of-band OSNR result.
Using the linear interpolation method (out-of-band measurement) gives no indication of the noise present at the actual channel wavelength, possibly resulting in an incorrect OSNR value. This direct spectral measurement using tunable filters cannot distinguish between coherent signal power and incoherent noise power. It must rely on baseline measurements at signal-free wavelengths, but such baseline wavelengths may not be present in networks with optical add/drop functions. Acquiring the true OSNR value requires an in-band OSNR measurement.
To measure the true OSNR, it is important to gain access to the in-band noise inside the optical transmission band of the ROADM filters. Current approaches that employ an in-band noise measurement are shoulder methods using software-based solutions with a conventional OSA and polarization diversity detection using limited polarization nulling techniques.
The shoulder method is based on the assumption that there will be a hump on either side of the optical signal channel peak indicating the noise shape of the optical filter. The noise is measured at this hump. In high-speed optical networks, the bandwidth of the optical signal will almost be as large as the filter bandwidth leading to a very smooth transition between the noise and the signal. In ROADM-based networks, multiple ROADMs are cascaded, leading to a narrowing of the filter shape and making it difficult to detect a hump for accurate noise measurement.
Another in-band OSNR measuring technique is based on a polarization nulling principle. An optical transmission signal channel comprises an arbitrary polarized light, whereas the Amplified Spontaneous Emission (ASE) noise comprises only non-polarized light. Employing an optical polarizer in the light path will either block or pass the optical signal depending on the input signal State of Polarization (SOP). A polarization splitter separates the input signal into two orthogonal polarization states suppressing the polarized transmission signal and passing the non-polarized noise. However, in WDM systems, the SOP of the optical signal varies from channel to channel. A polarization splitter provides a fixed SOP. The suppression of the polarized transmission signal depends on the matching of the SOP between the input signal and the polarization splitter. The suppression can therefore vary from channel to channel.
The shoulder method is inaccurate when measuring the in-band noise floor in high-speed ROADM-based networks. Polarization nulling technique is similarly unreliable, particularly when noise is partially polarized. Neither method is desirable to measure a true OSNR.
What is desired are systems and methods that measure the OSNR of a signal channel with high accuracy under all conditions, independent of data rate and modulation format.